The work opens by analyzing the profound shift in scientific method brought about by Darwin's Origin of Species, contrasting the modern pursuit of causes with the historical bugbear of speculation and the mere collecting of facts. The author establishes the core principles of evolution, arguing that natural descent is strongly recommended by the ubiquitous principle of continuity, while special creation relies on an invalid appeal to ignorance. Through an exhaustive study of natural classification, the text demonstrates that structural affinities and arborescent groupings reflect lines of genetic relationship and heredity rather than divine ideals.
Subsequent chapters delve deeply into morphology and vestigial structures, using examples ranging from whale flippers and hermit crabs to human anatomical anomalies to illustrate adaptive modification and obsolescence. Embryology is presented as a crucial witness, tracking individual development from unicellular ova through karyokinesis and gastrulation to show how ontogeny recapitulates phylogeny. The author then examines palæontological evidence, addressing the geological record's imperfections while highlighting remarkable progressive series in fossils, such as the evolutionary history of horse limbs, teeth, and mollusk shells.
Geographical distribution is thoroughly explored to show how geographical barriers and isolation correlate with organic types, drawing upon oceanic islands like the Azores, Bermudas, Galapagos, and St. Helena to demonstrate that every species arises coincident with a pre-existing, allied species. Finally, the text transitions into the mechanisms of evolution, contrasting the Lamarckian hypothesis of use and disuse with the revolutionary theory of natural selection. Throughout the treatise, the arguments build a cumulative, empirically grounded case for the transformation of species through natural causes.
